1. Field of the invention
The present invention concerns an alkaline electrolyte secondary electric cell, in particular a nickel-hydridable metal cell (Ni-MH).
2. Description of the Prior Art
The main advantage of secondary cells is their ability to store energy. However, a completely charged cell which is not used rapidly loses part of its charge. Nickel-cadmium (Ni-Cd) storage cells, for example, have long been used as an autonomous energy source. They are known to have good charge retention, i.e. when stored in the charged state the capacity falls slowly. The charge lost by a completely charged Ni-Cd cell is about 20% over 7 days at 40.degree. C.
Since modern portable appliances require even more powerful autonomous energy sources, a new cell has recently been developed. This is the nickel-hydridable metal storage cell (Ni-M). Such a storage cell has a specific energy which is at least equal to that of the Ni-Cd storage cell but the self-discharge rate is high and thus the user is greatly inconvenienced. The charge lost by a Ni-MH storage cell which is stored in its completely charged state is about twice as much as that for a Ni-Cd storage cell, i.e. 40% over 7 days at 40.degree. C. This poor result is due to the fact that the MH electrode, once charged, has a higher reducing character than the Cd electrode.
Self-discharge is generally attributed in part to nitrogen-containing shuttles. Ammonia and nitrites present in the storage cell are oxidized to nitrates at the positively charged electrode, discharging it. Also, nitrates and nitrites are reduced to ammonia at the negatively charged electrode, discharging that as well. Those reactions can occur a number of times since the species generated at the positive electrode will react at the negative electrode where they are transformed into species which are capable of reacting at the positive electrode. This is why they are called shuttles.
Reduction of nitrates and nitrites to ammonia is accelerated at an MH electrode. If such a reaction is the rate limiting step in the kinetics of the nitrogen-containing shuttle, then over a given period more shuttles can be produced in an Ni-MH storage cell. This hypothesis is generally accepted as the explanation for the high self-discharge rate in Ni-MH cells (Ikoma et al., J. Electrochem. Soc., 143, 6, 1996, 1904-1907).
In a cell, the positive and negative electrodes are separated by an insulative material which assures ionic conduction while preventing electrical contact between the two electrodes. In order to maintain electrical insulation between the two electrodes, the separator must be mechanically and chemically stable under service conditions. It must retain its properties during the entire service life of the cell. Further, a high ionic conductivity requires that the separator be uniformly wetted by the electrolyte.
In order to limit the influence of the nitrogen-containing species, it has been proposed to replace the polyamide separator generally used in Ni-MH storage cells by a polyolefin separator with a higher chemical stability in that medium. A polyamide separator is a potential source of nitrogen-containing impurities due to its deterioration in the highly alkaline electrolyte used in Ni-MH storage cells (U.S. Pat. No. 5,278,001).
The most frequently used separators at this time are based on polyethylene and/or polypropylene. Separators composed of fibers with a polypropylene core surrounded by a polyethylene sleeve are known in themselves, for example. However, polyethylene separators are difficult to wet in an aqueous electrolyte. In order to improve wettability, manufacturers have turned towards using separators which are grafted with hydrophilic monomers, which are generally vinyl compounds. The grafting method which has proved to be the most effective employs ionizing radiation, with the irradiation and grafting being carried out in two steps or simultaneously.
Such separators introduce no or very few nitrogen-containing species, but the other electrochemical components present in the cell remain unwanted sources of nitrogen-containing compounds. Thus self-discharge of a Ni-MH storage cell remains very much higher than that observed under the same conditions for a Ni-Cd storage cell.
The aim of the present invention is to propose a separator which increases charge retention in alkaline electrolyte secondary cells, in particular nickel-hydridable metal type cells.